1. Field of the Invention
The present invention concerns a procedure for controlling a scanning antenna, in order to transmit and/or receive at least one beam in a given tracking direction.
2. Description of the Background Art
Satellite-transmission networks for stationary ground users have been designed for creating telecommunications systems producing medium or high data rates, while meeting the following criteria:
use of small ground terminals (V.SAT); PA1 high total capacity of the system; PA1 high degree of efficacy of the radio-frequency-conversion device on board the satellite; PA1 dynamic allocation of the resource. PA1 coverage of the service zone by a plurality of beams, e.g., between six and twelve beams for Europe, PA1 increased capacity of the system by reuse of frequencies, through implementation of different linear polarizations, PA1 establishment of a variable capacity by using one or several temporal multiplexes per beam, although, in this case, granularity is weak, and PA1 time division multiple accesses (TDMA) to avoid blockage of the up-link. PA1 a) any point of the nominal synchronization zone can receive packets transmitted by the antenna when this point is located in a nominal coverage zone lying in at least one preferred direction and can ensure continuity of packet synchronization at reception when said point is not situated in one of said zones; and/or PA1 b) for any point of transmission located in the nominal synchronization zone, the antenna is capable of receiving packets transmitted by said point of transmission when it is located in a nominal coverage zone lying in at least one preferred direction and can ensure continuity of said packet synchronization at reception when said point of transmission is not located in one of said zones. PA1 a) any point of the nominal synchronization zone can receive packets transmitted by the antenna when this point is located in a nominal coverage zone lying in at least one preferred direction and can ensure continuity of packet synchronization at reception when said point is not situated in one of said zones; and/or PA1 b) for any point of transmission located in the nominal synchronization zone, the antenna is capable of receiving packets transmitted by said point of transmission when it is located in a nominal coverage zone lying in at least one preferred direction and can ensure continuity of said packet synchronization at reception when said point of transmission is not located in one of said zones. PA1 a central modulation-management unit incorporating an external angular scanning and packet-synchronization bus, PA1 at least N modulation-control devices, PA1 at least one beam-forming device, PA1 at least one network comprising M antenna elements, PA1 the central management unit incorporating N first terminals connected respectively to N second terminals of the beam-forming device through said N modulation-control devices, and the beam-forming device incorporating, on the one hand, M third terminals connected respectively to said M antenna elements, and, on the other, a scanning and synchronization terminal connected to said external bus. PA1 a central modulation-management unit containing an external packet-synchronization bus, PA1 M' groups of elements in series, each comprising a modulation-control device, a beam-forming device, a power amplifier, a Butler matrix, and a sub-group of P antenna elements, each modulation-control device having a terminal connected to a terminal on the central management unit, and each beam-forming device having a scanning/synchronization terminal connected to said external bus, PA1 and by the fact that it comprises a reflector working in conjunction with the N' sub-groups of P antenna elements in order to produce focused scanning.
These criteria entail complex compromises between the design of the multi-beam antenna, the transmission subsystem, and the switching and processing subsystem.
A number of known solutions yielding improved design of these systems include, in particular:
One solution also proposed involves the beam-jump technique, in which different packets of a temporal multiplex can be channelled to different beams, instead of being addressed continuously to a single beam. The disadvantage of this technique is that, in this instance, the link to the ground receiver becomes a link of the temporary division multiple access TDMA type (in contradistinction to a temporal multiplex TDM link), because of the fact that ground users work in the pulsed, and not the continuous, mode. This implies the use of more complicated demodulators and, furthermore, a reduction of the efficacy of the frame, since signalling flags must be added at the top of the frame to allow synchronization acquisition by the receiver.
Moreover, all of the aforementioned solutions presuppose the availability of fixed, predetermined beam positions, which overlap with a degree of overlapping of between 3 and 4.5 dB for most antennae. The required on-board radio/frequency RF power is determined by user demand in the EOC border coverage zones which "see" the lowest satellite antenna gain. The result is a loss of system output. Another output loss results from the granularity problem, i.e., the price which must be paid in power in order to increase the capacity of a beam by a quantity less than the transmission rate of a temporal multiplex.
In addition, a temporal multiple access (TDMA) implementing an antenna using multiple beams, each of which scans one dimension, has been described in the article of Anthony S. Acampora et al., in IEEE TRansactions on Communication Vol. COM27 No. 10, October 1979, entitled "A Satellite System with Limited Scan Spot Beams." In this system, each of the different beams scans parallel lines, and the beams are mutually isolated while alternating their polarizations. This system, of complicated design, allows coverage of a very large surface-area (all of the United States) using a relatively small number of beams, but at the cost of restricting very significantly accessibility to the device by ground receiving stations, because of the dilution of the time of access available to these stations resulting from scanning of lines of large amplitude.